1. Field of the Invention
This invention relates to a low-temperature method of fabricating a sintered nuclear fuel compact of uranium dioxide with a large grain diameter and more particularly, to an improved method of fabricating a sintered nuclear fuel compact which comprises preparing large grain size pellets stably by two-stage sintering in an oxidizing atmosphere at relatively different low temperatures and in a reducing atmosphere.
2. Description of the Related Art
A low-temperature sintering method for such nuclear fuel pellets is known, which can be carried out at a relatively low temperature on the order of 1100 to 1300° C., in contrast to a conventional sintering method of a nuclear fuel material which was conducted at around 1700° C. In general, low-temperature methods having recourse to a two-stage sintering, which includes sintering in an oxidizing atmosphere and subsequent reduction heating, have been widely known.
For example, JP Patent Publication 4(1992)-166800 A proposed by the same assignee of the present inventors discloses such a low-temperature fabrication method of a sintered nuclear fuel compact which comprises, preparatory to the aforesaid sintering in the oxidizing atmosphere, conducting a pre-heating in a main atmospheric gas of N2/air at a temperature in the vicinity of 150° C. or less to remove an excessive oxygen entrained upon discharging thereby dispersing appropriately the oxygen concentration in a compacted body, and subsequently conducting the sintering in an oxidizing atmosphere, namely in a main atmospheric gas composed of N2 of industrial purity and air, in which is added an oxygen concentration of 400 ppm or less for adjustment of the oxygen concentration, at a temperature of 1100 to 1300° C., followed by reduction heating at 1100 to 1300° C. in a main atmospheric gas composed of H2 or H2/N2 and 0.01 or upward of H2O in terms of volume ratio.
Here, the sintered nuclear fuel compact of uranium dioxide series thus obtained has generally an average crystal grain diameter of the order of 5 to 10 μm. Any sintered nuclear fuel compact of such a small diameter grain crystal is deemed to release a large amount of fission product gas (FR gas). In order to suppress the fission product gas, it is desired that a sintered nuclear fuel compact be a large grain size one having an average crystal grain diameter of ca. 20 to 60 μm.
On the other hand,, to fabricate a sintered uranium dioxide nuclear fuel compact having a large grain diameter as stated above, the sintering treatment is conducted by the use of a mire containing triuranium octaoxide (U3O8) obtained by sintering uranium dioxide scrap and uranium dioxide (UO2) fresh material powder. More specifically, for example, a method of fabricating a sintered nuclear fuel compact, whose O/U (oxygen/uranium) ratio is adjusted to 1.98 to 2.02, was also proposed by the same assignee and disclosed in JP Patent Publication 2(1990)-259496 A [JP Patent 7(1995)-31266 B], which comprises sintering a compact, obtained by compaction forming from mixed powers of 75 to 55% by weight of the uranium dioxide starting powder and 25 to 45% by weight of the triuranium octaoxide, in an oxidizing atmosphere at a temperature of 1100 to 1400° C. and subsequently heating it in a reducing atmosphere at a temperature of 1100 to 1400° C. In this method, the sintered nuclear fuel compact having a large grain size is produced by controlling the production condition of a raw material (UO2scrap) for triuranium octaoxide and the amount of the triuranium octaoxide to be mixed together thereby to regulate the density of the resulting sintered compact and the compaction degree upon sintering and by setting specific conditions such as a sintering temperature, etc. thereby to regulate the crystal grain diameter.
According to the foregoing method for fabricating a large grain size sintered compact, it is possible to obtain a sintered nuclear fuel compact having a grain diameter ranging from 20 to 60 μm and an average crystal density ranging from 93 to 98% TD (total density). In order to render the grain diameter as large as the above, however, many parameters are thus necessitated to be adjusted and a strict temperature regulation is required the control of which is too difficult to yield reliably and stably a sintered nuclear fuel compact of a large grain diameter.